2,732 research outputs found
Adsorption configurations and thermal chemistry of acetylene on the Ge(100) surface
No full textThe adsorption structures and thermal desorption behavior of C2H2 on Ge(100) were studied in ultrahigh vacuum by scanning tunneling microscopy (STM) and temperature programmed desorption (TPD). The STM investigation revealed that, at low coverage, C2H2 initially adsorbs onto the Ge(100) surface with two types of configurations: (i) a di-sigma configuration on top of a single Ge-Ge dimer (on-top) and (ii) a tetra-sigma configuration parallel to the dimer axes, bridging two neighboring Ge dimers (p-bridge). TPD measurements show that chemisorbed C2H2 desorbs from Ge(100) nondissociatively with two different desorption features, denoted as alpha (520 K) and beta (560 K). In addition, it was found that the desorption of C2H2 follows first order kinetics for both states and that the desorption energies of the alpha (520 K) and beta (560 K) states are 1.3 and 1.4 eV, respectively. STM studies of the adsorption of C2H2 at various Ge surface temperatures indicate that the alpha and beta features correspond to the on-top and p-bridge configurations, respectively.(C) 2002 American Institute of Physics.This research was supported by
KOSEF and Center for Nanotubes and Nanostructured Composites,
the Brain Korea 21 Project, and the Advanced Backbone
IT Technology Development Project of the Ministry of
Information and Communication
Surface dihydrides on Ge(100): A scanning tunneling microscopy study
No full textWe studied the atomic scale surface evolution of Ge(100) exposed at 300 K to gas-phase hydrogen atoms, H(g). Surface H(g) uptake created a 2x1:H phase, quickly reaching similar to1 monolayer H coverage. However, in contrast to the Si(100) surface, dangling bonds of the Ge(100) surface could never be completely removed by H(g) due to their regeneration by highly efficient surface H abstraction. This, together with the instability of surface dihydrides, GeH2(a), inhibited the large-scale formation of 3x1:H and 1x1:H phases. Short GeH2(a) rows, present in small metastable 3x1:H domains formed near defect sites, were etched selectively by H(g), producing line defects. (C) 2002 American Institute of Physics.This work is supported by KOSEF and the Center for
Nanotubes and Nanostructured Composites, the Brain Korea
21 Project, and the Advanced Backbone IT Technology Development
Project of the Ministry of Information and Communication.
One of the authors ~S.K.Jo! acknowledges funding
from the Korea Research Foundation ~KRF-2000-015-
DP0186!
Atomic-scale structural evolution of Ge(100) surfaces etched by H and D
No full textThe atomic-scale structural evolution of Ge(100) surfaces etched by H(g) and D(g) at T-s=400 K is studied using scanning tunneling microcopy (STM) and field emission-scanning electron microscopy (FE-SEM). The STM investigation reveals that etching of the Ge(100) by H(g) and D(g) proceeds initially via the production of single atom vacancies (SV), dimer vacancies (DV), and subsequently, line defects along the Ge dimer rows. It is also observed that D(g) etches the Ge(100) surface eight times faster than H(g) does. After extensive exposures of the surface to H(g), the FE-SEM images show square etch pits with V-groove shapes, indicating that H(g) etching of the Ge(100) surface proceeds anisotropically. (C) 2004 American Institute of Physics.KOSEF through the Center for Nanotubes and Nanostructured Composites, the Brain Korea 21 Project, the Advanced Backbone IT Technology Development Project, the National R&D Project for Nano Science and Technology
Ge adatom adsorption, diffusion, and exchange on surfactant-covered Si(111) surfaces
No full textWe investigate the effects of various surfactants on the Ge adsorption, diffusion, and exchange on Si(111) surfaces through first-principles pseudopotential total-energy calculations. For surfactant-covered surfaces such as Si(111):Ga-1 x 1, Si(111):As-1 x 1,and Si(111):Sb-(root 3 x root 3)R30 degrees, Ge adatoms are generally incorporated into the surfactant layer. On the Ga-covered surface, we find-strong interactions between the Ge and Ga atoms, which result in large activation energies for both Ge surface diffusion and exchange with an underlying Ga atom. In the case of As surfactants, the activation energies for adatom diffusion and exchange are much reduced because of the weak couplings between the Ge and As atoms. Similarly, on the Sb-covered surface, the exchange between the adatoms and surfactants takes place easily; however, the surface diffusion is severely suppressed due to a relatively large energy gain by the exchange process. [S0163-1829(99)00128-9]
Kinetics of H-2 (D-2) desorption from a Ge(100)-2x1 : H (D) surface studied using scanning tunneling microscopy and temperature programmed desorption
No full textThe kinetics of H-2 (D-2) desorption from a Ge(100)-2x1:H (D) surface was studied using scanning tunneling microscopy (STM) and temperature programmed desorption (TPD). Inspection of STM images of surfaces at the saturation coverage of H (D) (theta(H(D))similar or equal to1.0 ML) revealed a 2x1 monohydride (monodeuteride) phase in which most H (D) atoms were paired on Ge-dimers. By counting the sites of H-2 (D-2) desorption in STM images taken after desorption of H-2 (D-2) at temperatures in the range T-s=500-550 K, the desorption of H-2 (D-2) was found to follow first order kinetics with an activation energy of E-d=1.65+/-0.1 eV (1.65+/-0.1 eV) and a pre-exponential factor of nu(d)=(2.7+/-0.5)x10(13) s(-1) [(1.2+/-0.5)x10(13) s(-1)]. These values of E-d and nu(d) were used to simulate TPD spectra for the desorption of H-2 (D-2) from a Ge(100)-2x1:H (D) surface. The simulated spectra were in good agreement with the experimental TPD spectra. In contrast to the surfaces with saturated H coverage, which are characterized by pairs of H atoms on Ge-dimers, at the low H coverage of theta(H)similar or equal to0.05 ML unpaired H atoms as well as paired H atoms were observed on the Ge-dimers on the surface, causing the desorption process to follow second order kinetics. At T(s)similar to300 K, the singly occupied dimers (SODs) appear to be favored over doubly occupied dimers (DODs). However, upon increasing the temperature (T-s) from 300 to 500 K, most SODs were rapidly converted into the thermodynamically favored DODs by the migration of H atoms. On the other hand, it is observed that even above T(s)similar to500 K, the onset temperature for H-2 desorption from DODs, a non-negligible number of SODs remain on the surface due to the large entropic barrier to pairing. These results suggest that H adsorption in the low coverage is strongly influenced by the energetics of the pairing of H atoms. (C) 2003 American Institute of Physics.This research was supported by KOSEF and the Center
for Nanotubes and Nanostructured Composites, the Brain
Korea 21 Project, and the Advanced Backbone IT Technology
Development Project of the Ministry of Information and
Communicatio
Study of adsorption and decomposition of H2O on Ge(100)
No full textThe adsorption and decomposition of water on Ge(100) have been investigated using real-time scanning tunneling microscopy (STM) and density-functional theory (DFr) calculations. The STM results revealed two distinct adsorption features of H2O on Ge(100) corresponding to molecular adsorption and H-OH dissociative adsorption. In the molecular adsorption geometry, H2O, molecules are bound to the surface via Ge-O dative bonds between the O atom of H2O and the electrophillic down atom of the Ge dimer. In the dissociative adsorption geometry, the H,,O molecule dissociates into H and OH, which bind covalently to a Ge-Ge dimer on Ge(100) in an H-Ge-Ge-OH configuration. The DFF calculations showed that the dissociative adsorption geometry is more stable than the molecular adsorption geometry. This finding is consistent with the STM results, which showed that the dissociative product becomes dominant as the H2O coverage is increased. The simulated STM images agreed very well with the experimental images. In the real-time STM experiments, we also observed a structural transformation of the H2O molecule from the molecular adsorption to the dissociative adsorption geometry.the Brain Korea 21 Project, the SRC program (Center for Nanotubes and Nanostructured Composites) of MOST/KOSEF, the National
R&D Project for Nano Science and Technology, and Grants for Basic Research from the Korea Research Foundation
Adsorption and thermal stability of ethylene on Ge(100)
No full textWe have investigated the adsorption structures and thermal desorption behavior of C2H4 on Ge(100) using scanning tunneling microscopy (STM) and temperature programmed desorption (TPD) under ultrahigh vacuum (UHV). Ethylene molecules adsorb in two distinct bonding geometries: (i) on top of a single Ge-Ge dimer (on-top) and (ii) in a paired end-bridge between two neighboring Ge dimers within the same dimer row (paired end-bridge). Real-time STM images taken during the exposure of C2H4 to Ge(100) show that the on-top configuration dominates over the paired end-bridge configuration. The TPD measurements show that chemisorbed C2H4 desorbs from Ge(100) nondissociatively with two different desorption features, denoted as alpha (385 K) and beta (405 K). Desorption follows first-order kinetics for both states; the desorption energies of the alpha (385 K) and beta (405 K) states are 1.05 and 1.15 eV, respectively. These desorption energies are about 0.6 eV lower than those of ethylene on Si(100), indicating that the Ge-C bond is weaker than the Si-C bond. STM measurements carried out after annealing Ge surface at various temperatures indicate that the alpha and beta states correspond to the on-top and paired end-bridge configurations, respectively.KOSEF and the Center for Nanotubes
and Nanostructured Composites, the Brain Korea 21 Project, the Advanced Backbone IT Technology Development Project of the Ministry of Information and Communication, the National R&D Project for Nano Science and Technology
Double dative bond configuration: Pyrimidine on Ge(100)
No full textThe adsorption of pyrimidine onto Ge(100) surfaces has been investigated using real-time scanning tunneling microscopy (STM), temperature-programmed desorption (TPD), and density-functional theory (DFT) calculations. Our results show that the adsorbed pyrimidine molecules are tilted about 40degrees with respect to the Ge surface, and through a Lewis acid-base reaction form bridges between the down-Ge atoms of neighboring Ge dimer rows by double Ge-N dative bonding without loss of aromaticity. For coverages of pyrimidine up to 0.25 ML, a well-ordered c(4x2) structure results from states that appear in STM micrographs as oval-shaped protrusions, which correspond to pyrimidine molecules datively adsorbed on every other dimer. However, above 0.25 ML, the oval-shaped protrusions gradually change into brighter zigzag lines. At 0.50 ML, a p(2x2) structure results from the states that appear in STM as zigzag lines. The zigzag lines are formed by the attachment of pyrimidine molecules to the down-Ge atoms of every Ge dimer. However, the unstable p(2x2) structure eventually reconstructs into a c(4x2) structure due to steric hindrance between the adsorbed pyrimidine molecules after stopping the exposure of pyrimidine to the surface.KOSEF through the Center for Nanotubes and Nanostructured Composites, the Brain Korea 21 Project, National R&D Project for Nano Science and Technology, the Advanced
Backbone IT Technology Development Project of the Ministry of Information and Communication
Reactivity towards O-2 and H2O of the alkali-metal induced Ge(111)-3 x 1 surfaces: contrasting adsorption rates for different alkali metals
No full textUsing photoemission and Auger-electron spectroscopies, we investigated the reactivity towards oxygen and water of the alkali-metal induced Ge(I 11)-3 x I surface at room temperature. The adsorption of both gases on the Ge(I 11)3 x I surfaces induced by Na and K are significantly suppressed, but greatly enhanced on the Li-induced Ge(I 11)73 x I surface. This difference in the surface reactivity of the alkali-metal induced Ge(I 11)-3 x I surfaces contrasts not only to the expectation of the similar reactivity inferred from the common 3 x I reconstruction structure, but to the trends in the elemental alkali metals. It is proposed that the competition of the dangling-bond saturation and the AM-mediated oxidation determine the surface reactivity of the Ge(I 1 l)-3 x 1 surfaces. (C) 2003 Elsevier Science B.V. All rights reserved.This work was supported by the National R&D
Project for Nano Science and Technology, the Brain Korea 21 Project, KOSEF through the
ASSRC at Yonsei University, the Center for
Nanotubes and Nanostructured Composites,MOST and POSC
Hydrogen-surfactant mediated growth of Ge on Si(001)
No full textThe role of hydrogen in the growth of Ge on a Si(001)-(2 X I) surface was studied by scanning tunneling microscopy and medium energy ion scattering spectroscopy. The adsorbed hydrogen was found to (i) increase the number of equilibrium adsorption sites, (ii) lift the diffusion anisotropy, and (iii) lower the diffusivity for Ge adatom, as suggested by the recent first principle calculation. With a dynamically supplied atomic hydrogen flux of similar to 2 monolayers/s, we achieved layer-by-layer growth by preventing growth of the hut cluster beyond the known critical thickness. The 10.0 monolayer Ge layers grown with hydrogen surfactant are strained, while those without it are relaxed.We acknowledge financial support by Ministry of Science and Technology of Korea through
Creative Research Initiatives and Center for Molecular
Science
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